T6 Multivan gunk under front doors. What would be causing this?

[jets]

Pay particular attention to washing the lower part of the doors.

VW have been doing this for years, my'01 T4 still has some of this preservative under the doors. I would only clean off the wax that offends the eye & leave the rest to protect the panel from any future rust. We are fortunate in Aust. to own vans that are built to a higher standard of rust protection that is needed in Europe's harsher climate.